Medium-caliber autocannon barrels operate under thermo-mechanical loading regimes that combine high transient gas temperatures (1800–2700 K), peak chamber pressures of the order of 350–450 MPa, and cyclic mechanical contact between projectile driving bands and the rifled bore. Under accelerated firing regimes — bursts of 100–300 rounds at firing rates exceeding 200 rounds per minute — the resulting thermo-mechanical degradation accumulates non-linearly across the bore, thread, and chamber regions. The published 2017–2023 evidence base on gunbarrel degradation has matured substantially during this window, with sustained progress in coupled thermo-mechanical finite element modelling, validated material-testing data for 30SiMn2MoVA gun steel, and quantitative laws of bore-diameter evolution under continuous firing. Despite this maturation, no published study has supplied an integrated bedside-applicable degradation index that combines the three principal degradation mechanisms — thermo-mechanical fatigue, thermochemical-mechanical erosion-wear, and low-cycle fatigue — into a single composite remaining-service-life predictor specifically calibrated for medium-caliber autocannon geometries. This article, written with the benefit of the 2022–2023 cohort of validated thermomechanical models and the parallel maturation of the experimental literature on 30SiMn2MoVA degradation, fills that gap. The article introduces the Composite Bore Degradation Index (CBDI), a normalised 0–100 index constructed from three weighted sub-indices: a thermo-mechanical sub-index drawn from peak temperature and von Mises stress fields, an erosion-wear sub-index drawn from the diameter-change-rate law, and a low-cycle fatigue sub-index drawn from cumulative damage at the rifling start point. The CBDI is operationalised through a numerical workflow that combines coupled thermo-mechanical finite element simulation with the experimentally derived parameters of 30SiMn2MoVA steel and is applied to a generic 30 mm autocannon geometry across three accelerated firing scenarios. Three hypotheses are tested: that the dominant degradation mechanism in medium-caliber autocannons under accelerated firing is multi-mechanism rather than single-mechanism; that the CBDI's three sub-indices contribute non-uniformly to remaining-service-life prediction with the thermo-mechanical sub-index dominating in the first 2,000-round window; and that the CBDI offers actionable predictive accuracy that single-axis degradation metrics cannot replicate.